Mobile terminals are rapidly evolving from a simple phone with a camera to a powerful, multifunctional device, equipped with powerful processors, a large amount of memory, a high-resolution camera, multiple sensors, and a large touch-sensitive specialized display. At the same time, mobile terminals have a small form factor, which puts limitations on the size and shape of the battery. Even though current mobile terminals have a powerful battery, their capability of simultaneously running various applications, including real-time applications such as online games and streaming video and audio, imposes considerable restrictions on the amount of time that a mobile terminal can remain operational without recharging.
In the past, a mobile phone's performance was measured in terms of “talk time” and “standby” time between battery recharges, where the first measure indicates the total time a battery can power a mobile phone while it is used to perform calls, and the latter refers to the total time a battery can keep a phone operable. Additional performance parameters are currently being introduced to take into account the differences in power consumption per application, e.g., “Internet use time,” “video playback time,” and “audio playback time.”
However, if multiple services are used simultaneously, battery power is likely to drain rapidly, which will make it difficult to predict just how quickly the mobile terminal will run out of power. If the battery power is low, the user may try to reduce power consumption by avoiding or only briefly using certain applications. However, for many applications, it may be hard for the user to estimate and control the mobile terminal's power consumption, and some applications and services may operate in the background, making them even less visible to the user. Starting applications without knowledge of the actual power remaining in the battery may cause the mobile terminal to launch a power-intensive application when there is not much power left, thus quickly draining the remaining power and leaving the terminal inoperable until it is recharged.
Mobile terminals use highly integrated, low-power chipsets. The power consumption in chipsets, and in particular in processors, is largely determined by the supply voltage V, the clock frequency f, the fraction of gates actively switched a, and the leakage current Il. A processor's overall power consumption P is the sum of a dynamic power term and a static power loss term, and is generally modeled by P=aCV2f+VIl, where C denotes the capacitance load of the logic gates.
The processors used in mobile terminals typically have a very low static power loss. Large power savings can be obtained when the components are able to switch off internal modules when they are not in use. Processors may be designed to support dynamic frequency scaling, which provides a linear reduction of the dynamic power term. A quadratic reduction can be obtained if the supply voltage is dynamically adjustable. This is referred to as dynamic voltage scaling (DVS), which is one of the earliest approaches for power optimization. A lower supply voltage typically reduces the maximum achievable clock frequency, and therefore both the voltage and the clock frequency may be scaled down simultaneously to achieve significant power savings in exchange for computation speed.
Mobile terminals employ a wide variety of energy-saving strategies to limit power consumption, e.g., sleep modes and timers that switch the display to a low-intensity mode if it is inactive for a certain period of time. Some mobile terminals may provide an interface that allows the application to adjust the timers for low-intensity display. Mobile terminals typically use a battery charge status indicator and a reception-quality indicator to give the user an idea of the remaining power and the quality of the wireless connection, and they typically incorporate various sleep modes where parts of the system are switched off when there is no activity for a certain, pre-configured duration.
Larger mobile terminals such as laptops and notebooks typically have a power management function that warns the user when power is low. The power management function may take steps to save data so that an orderly shutdown of the system can be accomplished. The power management function may also switch off some functionality to conserve energy in sleep-mode.
The power management function is typically provided by the operating system. Laptop operating systems are typically desktop operating systems with two additional features: Wireless connectivity and user-controlled power saving features. Mobile operating systems, especially those used in multi-tasking smart phones, are often derived from the operating systems that are typically found in laptops.
The operating systems use task scheduling to maximize the utilization of the processor(s). In this context, we define a task to be the smallest unit which is scheduled for execution by the operating system. A process is an instance of a program that is executed to perform a designated job. A process may comprise one or more tasks that can be executed in any serial and/or parallel order. In some operating systems, a task is realized as a thread or a lightweight process.
In multi-tasking operating systems, tasks are assigned priorities for scheduling. Some tasks may be pre-empted to accommodate tasks with a higher priority. The goal of task scheduling in multi-tasking operating systems is to maximize the utilization of the processor(s). A wide variety of scheduling algorithms have been implemented in operating systems to achieve different performance criteria. In addition to processor utilization, other important criteria include fairness, throughput, turnaround time, waiting time and response time.
In battery-powered devices and computers where power consumption is critical, it is advantageous to consider power consumption as a further optimization criterion.
Power-aware algorithms for reducing energy use by applications have been proposed for use in mobile terminals. However, there remains a need for still greater awareness of actual power needs and actual capacity to deliver the needed power.